No one quite knows where the “urban legend ” about poinsettias being poisonous came from. It probably was started by the same joker who told the one about Daddy Longlegs being the most poisonous spider. First, Daddy Longlegs aren’t even an arachnid species. Next, they are NOT poisonous and neither is this lovely plant!
This plant is a member of the euphorbia genus–that means that it has a milky sap like all the other plants in that genus. You have probably seen it if you have broken off a large enough leaf or stem.
Some people might be bothered by this sap. I can’t say that I have known anyone who has reacted to it, but supposedly it is possible. It’s just something to know. I wouldn’t worry about it.
One thing to worry about is keeping these plants warm. They come from tropical climates so they need a warm room in your home, they need to be kept out of drafts, and by all means, if you live in the frozen north like I do, never leave them in a cold car! In fact, make sure to protect them with a sleeve or bag on the trip from the store to your home.
There are so many choices in colors and styles that there are Poinsettias for almost everyone. The only people who can’t grow them are, ironically, people like me, who keep their homes too cold.
So I buy them for others who I know will enjoy them–and who have warm enough homes. It’s called living vicariously.
Gardendaze 
I believe that the myth originated with the potentially caustic sap, and that it is related to species that produce sap that is even more caustic. Gophers do not dig through the roots of gopher purge, not because they are toxic, but because the sap is irritating to their very sensitive skin.
Tomatoes were considered by some to be toxic, since most related species really are quite toxic. Many edible fruits and vegetables are related to toxic plants.
I have heard–but did not verify–that a child got sick from eating the leaves. But the child merely got a stomach ache; he or she suffered no other ill effects.
I think if people realized that there are so many other truly toxic plants–like holly and mistletoe–that they might potentially have around in their homes this time of year, they would stop obsessing about the poinsettias.
It used to drive me nuts when I would be working in the shrub yard at the garden center and the people would start asking about every single plant and whether it was poisonous. I would ask them if they had azaleas and rhododendrons and they would say, “of course.” And I would reply, “if you’re truly concerned about poisonous plants, you’d better go home and rip them all out. Or you could just supervise your children and teach them not to eat the plants.”
Obviously my point was they couldn’t make everything safe so they’d better stop trying.
So, if you have a kid who chows down on house plants, maybe don’t bring a poinsettia home–but there are a whole host of other houseplants that are way worse than those–and you’d better figure out what they are, pronto!!
Karla